This invention relates to attitude control of spacecraft, and more particularly to the redistribution of net stored momentum in a spacecraft having four or more reaction wheels or momentum wheels.
Orbiting spacecraft are used for a large variety of sensing and communication purposes. For photographic purposes, it may be desirable for the spacecraft to be relatively near the Earth, so that the cameras or sensors are close to the subjects. For communication purposes, a geosynchronous equatorial orbit is often desirable. Whatever the orbit, a satellite must be stabilized in space if the sensors or antennas are to be pointed in appropriate directions.
Spacecraft attitude stabilization may be accomplished by spinning the spacecraft and by mounting the sensors or antennas on a despun platform. Alternatively, the spacecraft may be stabilized in three axes. Three-axis stabilization may be accomplished by a control system using fuel-burning thrusters, but the use of such thrusters requires the expenditure of fuel, which tends to limit the service life of the spacecraft. Another method for three-axis stabilization uses magnetic coils or torquers which interact with the magnetic fields of the heavenly body providing the desired torques. Magnetic torquers have the disadvantages that the available torques tend to be small, and undesirably dependent upon the local magnitude of the magnetic field of the heavenly body being orbited. The magnetic fields change from time to time and from location to location. The salient advantage of magnetic torquers, however, is that their operation requires only electrical energy, which may be a renewable resource on spacecraft equipped with solar panels.
Larger torques than those available by the use of magnetic torquers may be achieved with electrically driven reaction wheels or momentum wheels. Such wheels are also electrically driven and have the advantage of being able to provide relatively large torques regardless of orbital position.
In principle, a three-axis stabilized spacecraft requires only three mutually orthogonal reaction wheels or momentum wheels (referred to as "reaction wheels" or "wheels" hereinafter). In order to provide for redundancy in the event that one of three orthogonal wheels should fail, spacecraft often include at least one additional reaction wheel, oriented at a skew angle relative to the other three. The fourth wheel provides redundancy for all three wheels because it provides momentum components along the three axes. Thus, the skew reaction wheel may be used in conjunction with two of the other wheels to control the spacecraft attitude.
Increased expectations relating to the performance of spacecraft and improved capabilities have led to a continuing increase in the size of spacecraft. The increased size in turn requires greater torque and momentum capability along each control axis. Rather than use three larger mutually orthogonal reaction wheels with a skew wheel, it has been found that there are advantages to using four or more skewed smaller reaction wheels to obtain the required momentum and torque. When four or more reaction wheels are used, modern control techniques utilize all the wheels during operation.
When four or more reaction wheels are used for control, a given net momentum of the wheels may be achieved by many different wheel speed combinations, i.e., the three body momentum components are mapped into an infinite number of wheel momentum combinations.
During attitude control operations, the various reaction wheels are accelerated and decelerated to apply torques to the spacecraft body. For a given total spacecraft momentum, each wheel speed will drift from its optimal value, because of the under-determined nature of the wheel control, thereby increasing the total power consumption. In the worst case, a wheel may reach its maximum speed, even though the total wheel stored momentum is small.
A prior art speed management arrangement allows the wheel speeds to drift from their power optimal values and monitors the speed of each reaction wheel. When one of the reaction wheels reaches a predetermined speed threshold, it is shut off. This wheel's speed decreases to zero due to friction, and the wheel's momentum is redistributed to the other operational wheels. In this process a disturbance is applied to the spacecraft. This disturbance is the friction torque of the disabled wheel. This torque results in an attitude error.
A wheel speed management arrangement is desired that maintains each wheel at its power optimal speed without imparting a disturbance to the spacecraft.